According to US Patent Publication No. 2003/0150839, tapered (conical) holes 120-130 μm in diameter may be made by laser ablation followed by acid etching to remove surface defects and chips. The disclosed process requires an ion-exchange step before laser irradiation. Irradiation conditions beyond laser spot size and fluence are not disclosed.
US Patent Publication 2009/0013724 describes hole formation by laser irradiation and acid etching in glasses of various compositions. Lasers with wavelengths 355 nm and 266 nm were used. The recommended (numerical) beam aperture is NA<0.07 and the focus is disclosed as either within the glass or behind the back surface. Hole profile and placement accuracy are not specifically addressed.
A previously demonstrated process for making such dense arrays of holes in glass is disclosed in U.S. Application Ser. No. 61/418,152 filed Nov. 30, 2010, assigned to the present Applicant. The disclosed method involves glass exposure with a nanosecond laser with pulse frequencies in the from 5 to 50 kHz, in particular in the range of from 10 to 20 kHz. Holes with aspect ratio of up to 20:1 or more in glass of about 500 to 600 micrometer thickness can be formed in 80 to 100 milliseconds. The disclosed method reliably provides high quality holes. What is desirable is a relatively low-cost and reliable process for forming relatively small holes at relatively tight minimum pitch, with good positioning accuracy and reasonably small variation in diameter throughout the depth, with less laser exposure time than 80-100 milliseconds per hole.
According to the present disclosure, a method is provided for fabricating a high-density array of holes in glass at higher speeds than the previously described method. The improved method comprises providing a glass sheet having a front surface and irradiating the glass sheet with a laser beam so as to produce open holes extending into the glass sheet from the front surface of the glass sheet. The beam creates thermally induced residual stress within the glass around the holes—increasingly so as the pulse rate (and to some degree, power) of the beam is increased so as to increase hole drilling speed. After irradiating, the glass sheet is annealed to eliminate or reduce thermal stress caused by the step of irradiating, then the glass sheet is etched to produce the final hole size. Preferably, the glass sheet is also annealed before the step of irradiating, at sufficiently high temperature for a sufficient time to render the glass sheet dimensionally stable during the step of annealing after irradiating.
Variations of the method of the present disclosure are described in the text below and with reference to the figures, described in brief immediately below.